This invention relates in general to a method of repositioning teeth for use in orthodontic treatment. Particularly, this invention relates to the use of orthodontic appliances for producing tooth movements. More particularly, this invention relates to the use of a plurality of elastic repositioning appliances for producing such tooth movements.
Orthodontic treatments involve repositioning misaligned teeth and improving bite configurations for improved cosmetic appearance and dental function. Repositioning teeth is accomplished by applying controlled forces to the teeth over an extended period of time. This is conventionally accomplished by wearing what are commonly referred to as “braces.” Braces comprise a variety of appliances such as brackets, bands, archwires, ligatures, and O-rings. After they are bonded to the teeth, periodic meetings with the orthodontist are required to adjust the braces. This involves installing different archwires having different force-inducing properties or by replacing or tightening existing ligatures. Between meetings, the patient may be required to wear supplementary appliances, such as elastic bands or headgear, to supply additional or extraoral forces.
Although conventional braces are effective, they are often a tedious and time consuming process requiring many visits to the orthodontists office. Moreover, from a patient's perspective, they are unsightly and uncomfortable. Consequently, alternative orthodontic treatments have developed. A particularly promising approach relies on the use of elastic positioning appliances for realigning teeth. Such appliances comprise a thin shell of elastic material that generally conforms to a patient's teeth but is slightly out of alignment with the initial tooth configuration. Placement of the elastic positioner over the teeth applies controlled forces in specific locations to gradually move the teeth into the new configuration. Repetition of this process with successive appliances comprising new configurations eventually moves the teeth through a series of intermediate configurations to a final desired configuration. A full description of an exemplary elastic polymeric positioning appliance is described in U.S. Pat. No. 5,975,893, and in published PCT application WO 98/58596 which designates the United States and which is assigned to the assignee of the present invention. Both documents are incorporated by reference for all purposes.
In addition to their ease of use, polymeric positioning appliances are generally transparent, providing an improved cosmetic appearance, and impart substantial force on the teeth, due to stiffness of the appliance. The stiffness of an elastic positioning appliance is a result of the modulus of the thermoformable polymer materials from which it is made. The higher the modulus of the materials, the higher the stiffness of the appliance. When a patient positions such an appliance over a prescribed group of teeth, one or more of the teeth will provide a base or anchor region for holding the positioning appliance in place while the stiffness of the polymeric material will impart a resilient repositioning force against one or a portion of the remaining teeth. However, the stiffer the appliance, the more difficult it is to slip the misaligned appliance over the teeth and fully engage the appropriate surfaces; the appliance often has the tendency to disengage or “pop off”. Likewise, once it is firmly seated, it is more difficult to remove. Further, a stiff appliance is less forgiving in cases of lowered patient compliance. If a patient were to remove the appliance for an unprescribed period of treatment time, the patient's teeth may move slightly out of the planned tooth arrangement. When attempting to reapply the appliance, it may be too rigid to accommodate these slight differences and a new appliance may need to be created. Similarly, the tooth positions defined by the cavities in each successive appliance must not differ beyond a limiting dimension from those defined by the prior appliance or, again, it may be too rigid to accommodate the differences. Consequently, only small increments in tooth repositioning may be made with each appliance.
Thus, it would be desirable to provide tooth positioners, systems, and methods which apply adequate force to selected teeth yet overcome the inherent limitations of stiffness in the polymeric material. Likewise, it would be desirable to reduce the number of positioners required for a treatment plan by increasing the size of the repositioning increments throughout the plan. Further, it would be desirable to reduce the cost of lowered patient compliance by reducing the need for new appliances to be created for patient treatment resumption. At least some of these objectives will be met by the designs and methods of the present invention described hereinafter.